This invention relates to measurement devices and, in particular, to a solid state orientation sensor having a three hundred and sixty degree measurement capability for use in structural and biomedical applications.
Miniature orientation devices are used for a variety of structural and biomedical applications, including: measurement of structural angular displacement and orientation, computer input and pointing, virtual reality head and body tracking, camera stabilization, vehicle navigation, down hole drilling, feedback for functional electrical stimulation, and body position and inclination tracking. Sourced trackers use fixed magnetic field coils as a reference for magnetic sensors to detect position. (Raab et al., 1979) The source magnetic field coil is required to be relatively close (<10 feet) to the measurement coils. This greatly limit's these devices suitability in smart structure applications as it is often not practical to locate a source coil within this limited range. Sourceless trackers utilize earth's gravitational and magnetic field vectors, and do not limit a user's range of operation in any way.
This invention describes miniature, sourceless orientations sensor based on accelerometers and magnetometers that include analog and digital signal conditioning, embedded microprocessor, digital and analog output, and has the capability to measure pitch over a range of 360 degrees, yaw over a range of 360 degrees, and roll over a range of up to +/−90 degrees. Pitch, roll and yaw angles are computed in real time by a microprocessor located on the same board as the sensors, which eliminates the need for bulky external processing units, and facilitates networking.
The following prior art is known to the applicant:
U.S. Pat. No. 5,953,683 to Hansen et. al describes a number of devices that utilize linear accelerometers, magnetometers, and rate sensors to measure pitch roll and yaw. The device based only on accelerometers and magnetometers does not teach how to use the accelerometers to have a range of greater than +/−90 degrees of elevation or roll angles. Furthermore, the device does not utilize rate responsive adaptive filters that will be described in this text. The device also requires an initial calibration to determine the earth's magnetic field intensity however, our device does not require this because we use three axis of magnetometers and earth's total magnetic field intensity can be measured by the three magnetometers.
U.S. Pat. No. 5,373,857 to Travers et. al describes a sourceless tracker that utilizes an optical fluid based tilt sensor. This system has the disadvantage of being fluid based which leads to an undesirable settling time and cannot measure inclination angles that are greater than +/−70 degrees.